feat(otlp,sdk): add schema_url to tracer.

[TommyCpp]

part of #711

We have already introduced the schema_url parameters when we created the tracer. This PR pass it to the protos when exporting in OTLP format. See https://github.com/open-telemetry/opentelemetry-proto/blob/main/opentelemetry/proto/trace/v1/trace.proto#L71-L72

[jtescher]

@TommyCpp should we add this to the InstrumentationLibrary struct directly?

[codecov]

Codecov Report

Merging #743 (46e6a58) into main (104d66d) will increase coverage by 0.08%.
The diff coverage is 91.17%.

@@            Coverage Diff             @@
##             main     #743      +/-   ##
==========================================
+ Coverage   70.12%   70.20%   +0.08%     
==========================================
  Files         108      108              
  Lines        8789     8804      +15     
==========================================
+ Hits         6163     6181      +18     
+ Misses       2626     2623       -3     

Impacted Files	Coverage Δ
opentelemetry-api/src/metrics/observer.rs	60.00% <ø> (ø)
opentelemetry-sdk/src/trace/tracer.rs	90.81% <ø> (ø)
opentelemetry-api/src/metrics/config.rs	51.61% <50.00%> (+1.61%)	⬆️
opentelemetry-api/src/global/metrics.rs	83.33% <60.00%> (ø)
opentelemetry-api/src/common.rs	85.12% <100.00%> (+0.25%)	⬆️
opentelemetry-api/src/metrics/counter.rs	100.00% <100.00%> (ø)
opentelemetry-api/src/metrics/descriptor.rs	77.35% <100.00%> (+5.66%)	⬆️
opentelemetry-api/src/metrics/meter.rs	100.00% <100.00%> (ø)
opentelemetry-api/src/metrics/noop.rs	41.50% <100.00%> (ø)
opentelemetry-api/src/metrics/registry.rs	49.41% <100.00%> (ø)
... and 5 more

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[TommyCpp]

@TommyCpp should we add this to the InstrumentationLibrary struct directly?

Good point. But I think semantically it isn't used to describe the instrumentation library. Also, we will need to add it to Resource which doesn't have an instrumentation library so I feel it may be wired if we have schema_url as part of the instrumentation library in tracer but as a stand-alone attribute in Resource.

Maybe we can remove the instrumentation library as a struct and just use instrumentation_name and instrumentation_version? I can open a separate PR if that sounds more elegant.

[jtescher]

@TommyCpp other impls seem to add it to both InstrumentationLibrary as well as Resource, that's probably the simplest path. e.g. https://github.com/open-telemetry/opentelemetry-java/blob/main/sdk/common/src/main/java/io/opentelemetry/sdk/resources/ResourceBuilder.java#L21-L22 and https://github.com/open-telemetry/opentelemetry-go/blob/main/sdk/resource/resource.go#L35-L36